Furnace or gas hot water heater combustion exhaust gases are commonly vented through a chimney under the forces of natural air buoyancy. The natural tendency of hot gases is to rise. When hot rising flue gases are contained within a chimney, they tend to create a suction or reduced pressure condition at the base of the chimney. Under normal conditions this would sweep all of the products of combustion out of the dwelling via the chimney.
Under certain conditions within a dwelling the phenomenon of backdrafting may occur. Backdrafting is the reversed flow of air or gas through the chimney, from outdoors to indoors via the chimney. This occurs when the air pressure at the base of the chimney is less than the air pressure at the top of the chimney. In this condition outside air flows down the chimney into the dwelling and indoor air escapes through other openings, orifices or ventilation systems in the building envelope.
A low air pressure condition in a house can arise from forced air ventilation or from a separate fireplace that is drawing house air itself. Such a condition is more likely to arise, and be more pronounced, when the house is relatively air tight. Then the only major source for make-up air is commonly the furnace chimney. Backdrafting is also more likely to be persistent in a cold climate once the downward flow of relatively cold, dense outside air has thoroughly cooled the chimney and eliminated any tendency for the chimney to generate a natural upwards draft.
A chimney in a backdraft condition is not dangerous per se. But when it is coupled to a furnace, and particularly a gas furnace equipped with an open draft diverter air inlet, dangerous products of combustion are often diverted into the dwelling. Once a hot flow of exhaust gases into the dwelling has been established the backdraft flow of colder air down the chimney may not be overcome. The chimney may fail to re-establish its draft under conditions where the pressure at the base of the chimney is less than the weight of the column of cold air in the flue and chimney system. If the chimney is backdrafting the furnace exhaust gases will not exit via the chimney but will enter the dwelling via the draft diverter opening, reducing the indoor air quality level.
The phenomenon of backdrafting and resulting exhaust gas spillage into the dwelling can be complex to detect directly since direct detection would require the measurement of relative air pressures or gas flows. A cheap and effective means of detecting exhaust gas spillage is therefore desirable. In particular, it is desirable to identify a cheap and effective means for detecting the escape of combustion exhaust gas into a dwelling which has a chimney in a backdrafting condition.
A similar case of exhaust gas spillage into the dwelling occurs when the chimney pipe passage is blocked for any reason. Blockage of the chimney passage may occur as a result of faulty workmanship on the chimney flue or due to the accumulation of foreign matter such as animal nests or leaves anywhere along the flue and chimney passage. In some cases flue gases condense on the colder chimney liner leading to the buildup of corrosive liquids on the inner surface of the chimney pipe and liner. This corrosive liquid deteriorates the chimney passage leading to structural failure and blockage of the chimney passage.
The phenomenon of blocked chimney passages can also be difficult to detect directly since the direct detection requires measurement of the relative air pressures or gas flows.
Similar backdrafting conditions occur with naturally aspirated gas hot water heaters which also have an open draft diverter section.
In all cases, backdrafting exhaust gases into the home introduce undesirable combustion products into the indoor living space, and in some cases will lead to the build-up of hazardous carbon monoxide gas in the indoor living space.
It is common for naturally aspirated gas furnaces to backdraft for a short period, normally less than 30 seconds, during start up of the furnace. These initial transient backdraft conditions are not generally hazardous and if detected by a backdraft sensing means would constitute a false signal. Therefore, the detecting means must be resistant to such transient backdrafting conditions but remain sensitive to prolonged backdrafting conditions which tend to persist for more than 30 seconds.
The sensing means must also be designed to avoid false alarms from other normal conditions occurring within and near the furnace during normal operation, such as the normal emanation of heat from the furnace panels or the presence of hot gas flows within the proper flow passages in the furnace.